For a number of years governments have been struggling with how to keep terrorists from trafficking in special nuclear materials (SNM) and devices containing such materials and radiological dispersion devices (RDD). Such materials include weapon grade Uranium (WGU) and weapon grade Plutonium (WGP) and radioactive sources used for RDD. Such trafficking can take place by people, car, truck, container, rail, ship or other supply chain means. There is a long perceived need for a cost/effective system to screen, detect, locate and identify SNM or RDD materials or devices that are being transported. Furthermore there is a long felt need for an effective means to scan, locate and identify suspected areas in which those threats may be present.
Such screening is difficult in practice due, at least in part, to the environment in which it is done. Firstly, environmental radiation (including terrestrial and atmospheric radiation) of gamma rays and neutrons is substantial. Secondly, benign Normally Occurring Radiological Materials [NORM] like K-40 occur in nature and are present in many benign cargos. For example, kitty litter, plywood, concrete and bananas, emit substantial amounts of benign radiation. Additionally, humans undergoing nuclear medicine imaging or radiation treatment using implanted radioactive seeds can emit sizeable amounts of radiation. These and other “natural” or “benign” sources of radiation coupled with the ability to shield (using high Z materials like lead to shield gammas and some low Z materials to shield neutrons) the SNM and RDD, make simple detection schemes either ineffective in finding nuclear radiological threats or prone to a poor receiver operating characteristic (ROC), for example by having a large percentage of false positives.
Substantial numbers of false positives (also called ‘false alarms’) produce a large number of screened objects (e.g. vehicles, people, cargo) that have to be searched or otherwise vetted manually, making such simple systems practically useless for screening large numbers of objects. At present the leading means to screen RDD and SNM trafficking vehicles are the so called next generation Advanced Spectroscopic Portals (ASP) developed recently for the U.S. DHS DNDO.
More than 90% of the ASP systems use an array of 8 or 16 relatively small NaI(Tl) scintillators (e.g., 0.1×0.1×0.4 meter), to detect the gamma energy spectroscopic signatures of SNM and RDD, and a small array of He-3 Neutron detectors to detect and count neutron emissions.
ASP systems do not provide nuclear imaging, of either gamma rays or neutrons. ASP systems detection performance is limited primarily due to the high cost of NaI detectors, which limits the system detection area/sensitivity. Because of the high price and practical cost constraints of the NaI(Tl) and He-3 detectors, their number is small [typically the ASP NaI detectors have a sensitive area of 0.64 meter2] relative to the distance from the threat radiation source, resulting in a small solid angle of the detector as viewed by the threat. This limits the detection sensitivity and selectivity.
It is noted that while, for a given stand-off distance, the total detected radiation (benign radiation and the threat radiation) is proportional to the solid angle subtended by the detectors at the emitting radiation sources, the background radiation sigma (statistical standard deviation) is proportional to the square root of the solid angle. Thus, a 100 fold increase in solid angle (≈detector size) results in a 10 fold increase in detection certainty (number of standard deviations above the signal mean) to threats in a given screening condition. For example, if the small area (i.e. small solid angle) could reliably detect a source with 10 micro Curie of activity, the 100 times larger detector will detect 1 micro Curie with the same certainty (same rate of true and false detections, given the same geometry and background radiation).
Furthermore, the ASP detects only one threat signature for WGU and RDD—its gamma spectroscopic signature, since such materials do not emit neutrons in an amount much different from background. For WGP it detects also as a second signature its neutron emission. Having only one or two signature detection capabilities makes the system less reliable.
In addition, ASP systems do not provide several other SNM-RDD signatures such as 1D, 2D and 3D nuclear imaging, temporally based signatures such as cascade isotopes (e.g. Co60) doublets detection and gamma/neutron salvo emanating from spontaneous fission of SNM. Having such additional signatures would improve the ROC.
These and other limitations are known in the art and drove the DHS DNDO to publish the BAA-06-01 document. This publication states the need to come up with transformational technologies which will provide a much better than ASP SNM signatures detection performance, such as lower cost detectors, improved energy resolution detectors, the use of other than gamma energy spectroscopy SNM-signatures (e.g. spontaneous fission signature, imaging), detection of incident gamma or neutron directionality and other means that improve the overall system ROC.
The prior art teaches that organic scintillators (OS) provide a highly robust and stable material that is easily formable in many shapes, with the best detection sensitivity when cost per detected Gamma events is considered. On the other hand, there is a common belief in the prior art that organic scintillators, although some non-spectroscopic OS based portals have been used in the past, fail to provide acceptable ROC as they do not provide energy resolution (or at best a very limited one) in the context of nuclear threat detection. This explains why organic scintillators haven not been used for direct gamma spectroscopy isotope identification in nuclear radiological spectroscopic portals (NRSPs) (in the way NaI(Tl) and HPGe detectors are used in ASP) to identify and/or provide reliable energy window of SNM, RDD and NORM selected gamma energies. Furthermore, it is accepted that for all practical purposes screening portals organic scintillators have a poor gamma efficiency or “stopping power” at energies above 300 keV as compared to NaI(Tl). A review of this issue is given in: Stromswold, D.C. et al., “Comparison of plastic and NaI(Tl) scintillators for vehicle portal monitor applications” in: Nuclear Science Symposium Conference Record, 2003 IEEE, Vol (2) pp. 1065-1069. October 2003. The disclosure of this paper is incorporated herein by reference.
In recent studies related to anti-neutrino detection (see http.//arxiv.org/ftp/physics/papers/404/0404071.pdf) and in other publication of the same group (see F. Suekane et al., “An overview of the KamLAND 1; K-RCNP International School and mini-Workshop for Scintillating Crystals and their Applications in Particle and Nuclear Physics Nov., 17-18, 2003, KEK, Japan, it has been shown that extremely large (8 meter diameter) expensive (>$100 million, due mainly to the very large detector size and large number of large [18”] photomultiplier tubes (PMTs) used) liquid scintillator detectors can provide gamma energy resolution which is close to that of NaI(Tl). Such devices are not practical for large scale (or even small scale) deployment for threat detection due to their geometry and astronomical cost. The disclosure of this paper is incorporated herein by reference.
R. C. Byrd et al., in “Nuclear Detection to Prevent or Defeat Clandestine Nuclear Attack”, IEEE Sensors Journal, Vol. 5 No. 4, pp. 593-609, 2005, present a review of prior art of SNMRDD screening, detection and identification techniques. The disclosure of these papers is incorporated herein by reference.
In a PNNL report by Reeder, Paul L. et al., “Progress Report for the Advanced Large-Area Plastic Scintillator (ALPS) Project: FY 2003 Final” PNNL-14490, 2003, a PVT light collection efficiency of 40% for a 127 cm long detector is described. It should be noted that a straight forward extension to 4 meters length of the PNNL OS approach would have resulted in less than 25% light collection and less than 15% light collection for a 6 m long detector. The disclosure of the PNNL report is incorporated herein by reference.
The above referenced patent publications describe a number of embodiments that ameliorate some or all of these problems. For example, these publications describe a number of structures to detect radiation particles, such as those emitted by nuclear threats with increased efficiency and spectral purity. Some embodiments utilize thick plastic or liquid scintillator materials to increase the capture efficiency and allow for more accurate determination of the captured radiation particles. In general the energy in the particles is captured in a number of interactions, in which the radiation gives up energy converted into light scintillations. As mentioned therein, despite the thickness of the detector, for some particles, a portion of the energy is not captured due to what are described as “escape quanta”, namely uncaptured secondary radiation which escapes from the detector. U.S. 2006/0289775 mentions in paragraph [214] that it is possible to discriminate particles that do not give up all their energy based on the number of interactions that take place and result in scintillations.
Further information on the state of the art can be found in the Background section of and referenced prior art listed and included by reference in the above referenced U.S. patent application and provisional patent applications.